Plant yield is a very complex trait involving on a molecular basis the interaction of many pathways and interacting factors. There has been a tremendous amount of focus in the field of commercial agriculture over the past decade to develop higher yielding plants either through traditional plant breeding or genetic modifications. In a simplified view, the yield of a plant ultimately depends on the energy the plant gains through fixing carbon dioxide into carbohydrates during photosynthesis. The primary plant tissue involved in photosynthesis are the leaves and to a lesser extent the stem tissue. All other tissues such as the roots and seed are dependent on the photoassimilates made in the photosynthetic tissue. In general, this can be seen as an energy flow from photosynthetically active tissues to photosynthetically inactive tissues.
Phloem transport of this energy is determined by the relative locations of the areas of supply and utilization of the products of photosynthesis. Translocation occurs from areas of supply (sources) to areas of metabolism of storage (sinks). Sources include any exporting organ, typically a mature leaf that is capable of producing photosynthate in excess of its own needs. The direction of phloem transport of this energy is determined by the relative locations of the areas of supply and utilization of the products of photosynthesis. Another type of source is a storage organ during the exporting phase of its development. For example, a storage root may be a sink during the first growing season when it accumulates sugars received from the source leaves. During the second growing season the same root could become a source, when the stored sugars are remobilized and utilized to produce a new shoot which ultimately becomes reproductive. Sinks include any non-photosynthetic organs of the plant and organs that do not produce enough photosynthetic products to support their own growth or storage needs. Roots, tubers, developing fruits and immature leaves which must import carbohydrate for normal development are all examples of sink tissues. Sink tissues differ in their ability to attract source products. Elements such as stress, developmental stages of plant tissues, and osmotic potential all may affect the transport of photoassimilates.
Differential distribution of photoassimilates within the plant is termed partitioning. Partitioning of assimilated carbon amongst sink organs is a critical factor that controls rate and pattern of plant growth. The regulation of the diversion of fixed carbon into the various metabolic pathways is termed allocation. The rate of fixed carbon in a source cell can be classified into three principle categories; storage, utilization, and transport. Starch is synthesized and stored within chloroplasts and in most species is a primary storage form that is mobilized for translocation during the night. Fixed carbon can be utilized within various compartments of the photosynthesizing cell to meet energy needs of the cell or provide carbon skeletons for the synthesis of other compounds required by the cells. Fixed carbon can also be incorporated into transport sugars for export to various sink tissues.
The rate of photosynthesis of leaves is strongly influenced by the demands of the sink. There are cases in which senescent leaves can be rejuvenated to full photosynthetic performance when the sink/source ratio is increased substantially. On the other hand rapid growth of a sink can sometimes compete with leaves for remobilizable nitrogen leading to senescence of the leaf and a drop in its photosynthetic capacity. Young leaves normally act as a sink rather than as a source. After a certain time however they begin to export carbohydrates to the phloem although import carbohydrate may continue for a while through different vascular strands. Once sucrose begins to actively load into companion cells and then into the sieve elements, water will enter by osmosis and flow will begin out of the line of veins. The leaf will become a source instead of a sink.
Two primary photoassimilates are sugar and starch, and these products are important to yield and plant development. Sugar and starch biochemistry are interrelated in plants. (See, e.g., Sivak, M. N. and J. Preiss (1994). Starch synthesis in seeds. In: Seed development and germination. Kigel, J. and G. Galili, eds. (Marcel Dekker, New York), pp. 139-168; J. S. Hawker (1985). Sucrose. In: Biochemistry of storage carbohydrates in green plants. P. M. Dey and R. A. Dixon, Eds., (Academic Press, London), pp. 1-51, which are incorporated herein by reference).
During the early development of storage organs, such as seeds and tubers, sucrose is imported and used for building the cellular components required for growth and development. Following this phase the metabolic program changes to convert the imported sucrose into storage compounds such as starch in tubers and fatty acids in oil seeds. Metabolism is finally altered to convert the starch and oils into reduced carbon compounds for the development of sprouts and seedlings respectively. Sucrose levels rise when hexoses decrease apparently terminating cell division in initiating differentiation and storage activities.
Early ear development in species belonging to the grass family Poaceae (e.g. wheat, maize, wheat, etc) relies upon concurrent photosynthate transport into reproductive sink tissue, as the developing seed cannot utilize stored photoassimilates present in other plant tissues. Because the seed are weak sinks, it is unable to attract stored reserves from source tissues. Seed abortion may occur when concurrent photosynthate is insufficient to meet the needs of reproductive growth, resulting in dramatically decreased yield, or in the case of maize ear, barreness. Anthesis is generally recognized as the critical period of ear and kernel development. Varied experimental approaches have demonstrated that treatments, which decrease the plant carbon exchange rate (CER) around anthesis, decrease grain yield. For example, large yield losses occur when maize plants are shaded (Early et al., 1967; Schussler and Westgate, 1991; Andrade et al., 1993), defoliated (Tollenaar and Daynard, 1978), subjected to water-deficits (Denmead and Shaw, 1960; Claassen and Shaw, 1970; Moss and Downey, 1971; Westgate and Boyer, 1986; Schussler and Westgate, 1991) or exposed to high plant density (Prine, 1971; Baenziger and Glover, 1980) around anthesis. Conversely, treatments that increase plant CER around anthesis increase grain yield. For example, yield enhancements are obtained when maize plants are provided supplemental radiation (Schoper et al., 1982; Ottman and Welch, 1988). In all cases, the variation in yield was directly related to the number of kernels that developed and supply of concurrent photosynthate. Collectively, these results suggest that kernel number may be limited by carbohydrate supply, particularly during drought stress at anthesis.
One aspect that has not been looked at in detail, is whether or not metabolic profiling in these source and/or sink tissues may give insight into the more subtle processes such as sugar and stress signaling. Metabolic profiling has become a powerful tool in the discovery of processes, interactions, compounds and pathways that might be involved in various biological phenomena. According to the invention, further exploration into metabolite profiling of reproductive sink tissue has led to some unexpected findings that certain metabolite profiles can be utilized to make a determination as to predicting yield of an individual plant. Accordingly, data from single plants can be further statistically validated to predict the yield of a given plant population. The ability to correlate yield with the presence or absence of specific metabolites and/or combinations thereof in reproductive sink tissue have many useful applications that will be described further herein.